The present invention relates generally to improved electronic flash apparatus. In particular, the present invention is concerned with control systems for controlling the charging of the flash storage capacitor of electronic flash apparatus.
Electronic flash apparatus is known in the art in which a relatively high voltage is stored on a capacitor. At the instant of taking a picture, the capacitor is discharged through a gas filled flash tube to provide the desired light.
In electronic flash apparatus of this type, some form of charging apparatus or power supply must be provided to recharge the flash storage capacitor. One particularly advantageous charging apparatus is shown in U.S. Pat. No. 3,863,128 by Dennis J. Wilwerding, which is assigned to the same assignee as the present application. In this system, a battery powered DC to DC oscillator charges the flash storage capacitor of the electronic flash apparatus. Monitoring and controlling circuits are provided to maintain the voltage on the flash storage capacitor within desired limits.
Certain electronic flash apparatus include separate capacitor charging and flash units or modules. By separating the charging unit from the flash unit, it is possible to use the same charging unit to charge the flash storage capacitor in several different flash units. In addition, the flash unit can be lighter and more compact when the charging unit is separate.
Electronic flash apparatus having separate charging and flash units preferably has the flash storage capacitor in the flash unit rather than in the charging unit. This reduces the current carrying requirements for the wires connecting the charging unit to the flash unit. Very high currents are produced when the flash storage capacitor is discharged through the flash tube to produce a light flash.
One disadvantage of having the flash storage capacitor in the flash unit is that the charging unit cannot directly sense the voltage on the flash storage capacitor. The voltage on the flash storage capacitor cannot be directly sensed because a pair of diodes are connected between the charging unit and the flash storage capacitor to allow only unidirectional current flow from the charging unit to the flash storage capacitor. The diodes, which are connected between one terminal of the flash unit and the flash storage capacitor, prevent a discharge of the flash storage capacitor if the user accidently comes in contact with both terminals of the flash unit.
In this type of electronic flash apparatus, the separate charging unit contains an internal capacitor and resistor which will discharge with the time constant determined by the capacitance and resistance or will follow the voltage on the flash storage capacitor as the flash storage capacitor discharges to any voltage below that on the internal capacitor. When the voltage on the internal capacitor decreases to a predetermined value (generally about 200 volts) the oscillator turns on and charges the internal capacitor and the main storage capacitor to the desired maximum voltage (generally about 350 volts). The charging unit then turns off until the voltage on the internal capacitor has again reduced to the predetermined minimum value.
Although generally satisfactory, this system does allow the voltage on the flash storage capacitor to decrease significantly without recharging. For example, in automatic electronic flash apparatus of the series termination type, the light flash may be terminated before the flash storage capacitor has been fully discharged. Unless the voltage on the flash storage capacitor has decreased from its maximum desired value of about 350 volts to about 200 volts, it will not be recharged even though a light flash has been produced.
For example, a terminated flash may cause the voltage on the flash storage capacitor to drop by only about 20 volts. Although the voltage on the flash storage capacitor has dropped, it has not decreased enough to turn on the charging unit. The next light flash will be produced, therefore, with the voltage on the flash storage capacitor at a value which is less than the maximum desired voltage.